Isostress grid array and method of fabrication thereof

ABSTRACT

An electronic device package includes a substrate and wire columns arranged in groups about a neutral stress point of the substrate. The height of the wire columns is substantially uniform for the plural groups of wire columns, and a length of at least one of the wire columns is greater than the uniform height. A method of fabricating an electronic device package having a column grid array includes applying two templates on wire columns of the column grid array and bending at least one wire column to increase its length while maintaining a uniform height for the column grid array. In another aspect, an electronic device package substrate includes wire columns having at least one non-uniformity in lengths of the columns, and the length of a wire column corresponds to a distance of that wire column from the neutral stress point of the substrate. The non-uniformity of length in the wire columns reduces stress in the package leads after attachment of the package to a carrier substrate, such as a printed circuit board.

RELATED APPLICATION

This disclosure claims priority under 35 U.S.C. §119 to U.S. ProvisionalApplication No. 61/272,484 filed Sep. 29, 2009, the content of which isincorporated herein by reference in its entirety.

FIELD

An electronic device package including a column grid array, and a methodof fabricating an electronic device package having a column grid array,are disclosed.

BACKGROUND

High input/output (I/O) count electronic device packages can have anintegrated circuit (IC) bonded to one side of a substrate and an arrayof metal connectors extending from the opposite side of the substrate.The electrical connectors of the array can be soldered or welded to acarrier substrate, such as a printed circuit board (PCB).

Two known types of electronic device package connection configurationsare ball grid array (BGA) and column grid array (CGA). BGA utilizessmall solder balls that are arranged into an array on the package'ssubstrate, and can, for example, be used for small chip applications orapplications having a well-characterized operating environment. CGAutilizes solder columns that are taller and have less tin content thansolder balls of BGA, which provide a more compliant and flexible I/Oconnection points that can withstand large temperature or mechanicalfluctuations. CGA can provide long life and high reliability, and can beused for in high-performance applications.

SUMMARY

An exemplary electronic device package is disclosed which includes asubstrate and wire columns arranged in plural groups about a neutralstress point of the substrate. The height of each of the wire columns issubstantially uniform for the plural groups of wire columns, and alength of the wire columns increases for each group as a function ofeach group's distance from the neutral stress point.

An exemplary method for fabricating an electronic device package havinga substrate and a column grid array includes applying wire columns ofthe column grid array in a first direction through holes in a firstremovable template such that portions of the wires protrude from thefirst removable template to provide a uniform distance between the firstremovable template and the substrate. The wire columns protruding fromfirst removable template are applied to a second removable template. Atleast one of the wire columns in bent to increase the length of a wirecolumn between the first removable template and said substrate whilemaintaining the uniform distance.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings provide a further understanding of exemplaryembodiments and are incorporated in and constitute a part of thisspecification. The drawings, with the description serve to explain theprinciples of the invention. In the drawings:

FIG. 1A is a diagram showing a bottom view of a column grid arraypackage in accordance with an exemplary embodiment;

FIG. 1B is a diagram of a side view of a column grid array packageaccording to an exemplary embodiment; and

FIG. 2 is a diagram showing a column bending operation in accordancewith an exemplary embodiment.

DETAILED DESCRIPTION

FIG. 1A shows a bottom view of an electronic device package 100according to an exemplary embodiment. The electronic device package 100includes a column grid array comprising wire columns 112, 116 and 120arranged on one side of a substrate 101. On the opposite side of thesubstrate, an IC chip or other electronic device can be bonded to thesubstrate 101. Interconnections electrically connected to the electronicdevice can be formed through the substrate 101 to plural contact pads onthe opposite side of the substrate 101, and the wire columns 112, 116and 120 can be attached to these contact pads.

The substrate 101 can comprise a material having a coefficient ofthermal expansion different from a carrier substrate that the columngrid array package 100 can be attached to, such as printed circuitboard. For example, the substrate 101 can comprise a material such as aceramic material or silicon, and the carrier substrate material cancomprise a polymer or another thermally mismatched material. During athermal event, the mismatch between the coefficients of thermalexpansion of the substrate 101 and the carrier substrate causes thesematerials to expand at rates different from one another. Frequenttemperature cycling over time can cause multiple cycles of expansion andcontraction, leading to cumulative stress and strain effects on thesolder material, which can eventually lead to failure at a wire columnattachment point. In aggressive environments, thermal cycling can occureven more frequently, causing devices to fail at a correspondingaccelerated rate.

The length of wire columns can be increased to reduce strain duringthermal expansion because the strain is inversely proportional to wirecolumn length, and a reduction in strain, in turn, reduces stress.However, increasing the length of each of the wire columns cancorrespondingly increase susceptibility to vibration, and at some point,the length cannot be increased further to improve fatigue life withoutintroducing unacceptable vibration. Additionally, standoff distancerequirements between the package substrate 101 and the carrier substratemay prevent simple increases in column length beyond a specified value.

Embodiments described herein can reduce stress in a wire column whilemaintaining a specified height by bending the column one mode past thebuckling mode appropriate for that wire column's distance from theneutral stress point of the package. As referenced herein, a “neutralstress point” is a point on the electronic device package that wouldexhibit little or no movement relative to a carrier substrate when theelectronic device package is attached to the carrier substrate andsubjected to thermal cycling. The location of the neutral stress pointcan be, for example, close or even very close to the geometric center ofan electronic device package substrate for electronic device packageshaving a symmetrical shape or symmetry in wire column arrangement. Forexample, the embodiment of FIG. 1A, the substrate 101 includes a neutralstress point 108 located in the center of the substrate 101, and thewire columns 112, 116 and 120 are arranged about the neutral stresspoint 108.

Wire columns can deform through strain hysteresis over the fatigue lifeof an electronic device package in fixed buckling modes. The stressimposed on a column grid array of an electronic device package fromthermal expansion mismatches can be greater at outer points of the arraythan areas closer to the neutral stress point because these outer pointsexpand more from the cumulative effects of expansion of inner areas. Thestrain (proportional to stress) experienced by a column can be afunction of that column's distance from the neutral stress point of theelectronic device package. As the distance from the center of the arrayincreases in any direction, the strain due to thermal cycling andimposed by column expansion increases until the outermost columnsexperience the most strain and undergo the highest fatigue or failure.

These failure rates can be described, for example, by the Coffin-Mansonequation by relating elements of temperature, frequency of cycling, andapplied stress due to thermal expansion. The equation can be stated torelate the number of cycles of fatigue as being proportional to theamount of stress in the columns experiencing a thermal cycle. Thisimplies that the amount of stress experienced is inversely proportionalto the length of the column experiencing stress. As a result, thecolumns in the center of an array can comprise a smaller length thancolumns at the edges of the array. These columns can be divided intogroups, with each group having substantially the same length. Thislength can increase as the distance of a group from the neutral stresspoint of the column array increases.

Referring now to FIG. 1B, a side view is shown of the exemplaryelectronic device package 100 embodiment taken along the section A-A ofFIG. 1A. FIG. 1B illustrates a substantially uniform height h (e.g.,heights within ±10%, or greater or lesser), for example, correspondingto a standoff distance, between the package substrate 101 and a carriersubstrate. While the height h is essentially uniform for all the wirecolumns 112, 116 and 120, bends can be included in at least one of thewire columns to increase the lengths of that column. More particularly,FIG. 1B shows details of an exemplary embodiment including a bentportion 116 b between end portion 116 a and 116 c of a wire column 116,and a bent portion 120 b between end portions 120 a and 120 c of a wirecolumn 120.

Although FIGS. 1A and 1B show an embodiment of an electronic devicepackage 100 having groups of wire columns with different sized bentportions, it will be appreciated that embodiments can include wirecolumns having one or more bends of only one size, or that otherembodiments can have all the wires columns bent to one size. Also, whilethe wire column sections 116 b and 120 b are depicted in the exemplaryembodiment of FIG. 1A at an angle of about 45 degrees from a horizontaldirection across the figure, which would allow use of greater sizedbends for a particular wire column pitch, it is to be understood thatembodiments can include columns that are bent only in one directionand/or at any orientation with respect to one another and the packagesubstrate.

Exemplary embodiments can include wire columns lengths that increase asa function of a wire column's distance from the package's neutral stresspoint. This non-uniformity in column length can provide a more uniformstress gradient across the array. Embodiments can includecontinuous-like increases in wire column lengths, where each columnlength is proportional to its distance from the neutral stress point.Other embodiments can include wire columns sectioned into plural groups.Wire columns in each group have a uniform height sufficient to maintaina standoff distance between the package substrate and a carriersubstrate; and a uniform length that corresponds to a buckling modequantized for that group, wherein the length of the wire columns isdifferent between at least two of the groups. The quantization of thebuckling modes for plural groups can be determined in a practical andconvenient way for ease of manufacture.

For example, FIGS. 1A and 1B show a group 102 of wire columns 112 withinthe area defined by dashed line 110, which is nearest to the neutralstress point 108. Each of the wire columns 112 has no bent portion, andthus the length of each column 112 is equal to the height h. The nextgroup 103 of wire columns 116 is located between dashed lines 110 and114. Each column 116 has a bend 116 b to provide a uniform increase inlength from the wire columns 112. In a similar fashion, each wire columnin a group 104 on the periphery of the package substrate 101 outside thedashed line 114 includes a bent portion 120 b larger than the bentportion 116 b such that the length of each wire column 120 is more thanthe length of a wire column 116.

The groups 103 and 104 shown in FIGS. 1A and 1B can be comprised ofbands, with one band 104 formed about another band 103. Groups also canhave any type of shape, such as circular, ring, annular, oblong,elliptical, rectangular, or another type of shape.

Other embodiments can include different types of groupingconfigurations. For example, package embodiments that do notsignificantly expand in one direction, such as rectangular shapedpackages, can have subgroups sections one group formed in anon-contiguous manner. Another exemplary non-contiguous embodiment caninclude a circular grouping scheme, with each of the four corners of thepackage substrate 101 including a subgroup of wire columns that arelonger than wire columns of a neighboring group formed closer to theneutral stress point 108.

FIG. 1B shows two exemplary bend geometries for a wire column and theheight of the wire columns, h, which is the linear distance between endpoints of the wire columns. It is to be understood that FIG. 1B shows anembodiment including profiles of the wire columns 116 and 120 of FIG. 1Arotated to be aligned with a horizontal direction of the drawing to showthe distances d1 and d2 respectively associated with the bend geometriesof the wire column portions 116 b and 120 b. In an exemplary embodiment,the height h of each wire column can be about 0.0870 inches, forexample, as a standoff height, although the height may be more or lessthan this value. For example, the wire column 116 can have a distance d1of about 0.0646 inches, and the length of the wire column can be about0.1012 inches (e.g., ±10%). Hence, the bend in the wire column portion116 b increases the wire column distance 16.3% from the height h. Thewire column 120 can have a distance d2 of about 0.0900 inches and thelength of the wire column can be about 0.1512 inches, which results in awire length increase of 73.8% from the height h.

While the depicted embodiment shows a bend can be in the shape of a “C,”bends in other embodiments can form another shape or configuration(e.g., the shape of an “S” or a spiral shape), which can depend on howmuch length the column will need based on the distance from the neutralpoint, the pitch if the wire columns etc.

With reference now to FIG. 2, there is shown a process of fabricatingbends in a column grid array of an electronic device package 200according to an exemplary embodiment. As shown in FIG. 2, electronicdevice package 200 includes a substrate 201 that having a grid array ofwire columns provided on one of its surfaces. The wire columns areinitially much longer than a standoff height h to provide adequatelength for forming the largest bends of the package while maintainingthe height h.

The wire columns are applied through respective holes in one side of atemplate 205 a (or cap piece), such that portions of the wire columnsprotrude from an opposite side of the template 205 a. The template 205 ais positioned a distance h from the substrate 201. To maintain theheight h, the template 205 a can include appropriately sized standoffs,or removable spacers of appropriate size can be provided between thetemplate 205 and the substrate 201.

Next, the portions of the wires protruding from the template 205 a areapplied through respective holes in one side of a template 205 b (or cappiece), such that portions of the wire columns protrude from an oppositeside of the second template 205 b.

After the templates 205 a and 205 b are in place on the wire columns,the length of one or more of the wire column can be increased within thearea defined by the template 205 a and the substrate 201. As shown inFIG. 2, a wire bending mechanism 210, such as a rod, bar or hook, can beprovided next to the column to be bent and moved to create a bend in thecolumn. For example, the bending mechanism can be moved in a directionparallel with the surface of the substrate 201 to form “C” shapedportions, such as portions 216 b and 220 b. For greater efficiency anduniformity, several wire columns can be bent simultaneously. In someembodiments, different sized bends can be formed selectively fordifferent groups of wire columns, such as the groups 202, 203 and 204depicted in FIG. 2. As described herein, the bend size can be determinedbased on a distance from a neutral stress point 208 of the substrate201.

After removal of the template 205 b, a column-cutting knife can be usedto cut off portions of the wire columns protruding from the firsttemplate or cap 205 a to provide the array with a uniform height h andsuitable wire column surfaces for attaching to an underlying carriersubstrate.

The electronic device package substrate 200 can be a ceramic material,although other types of materials, such as a polymer can be used. Thewire columns of a column grid array can be solder material, for example,low tin solder wire compositions such as a 90Pb/10Sn mixture or a highlead solder core mixture wrapped with a copper ribbon. These high lead(i.e., low tin) compositions can increase the degree of flexibility inthe column. The bending mechanism 210 can be a rod, bar, dowel or hook,or any other type of elongated device capable of creating bends in wirecolumns, and can have a cross section shape, such as a circular square,ellipse or ovoid, which facilitates creating a desired type of bend.

While embodiments described herein include the wire columns distributedevenly in an array on a package substrate, other embodiments can includewire columns concentrated about one or more package substrate areas, forexample, the center of the package substrate or at the edges of thepackage substrate. Additionally, a wire column group can have a shapeother than the square band shape shown in FIG. 1, for example,polygonal, annular, circular or irregular shape.

Also, while the FIG. 1 embodiment shows an electronic device packagesubstrate having a square shape, it is to be understood that otherembodiments of an electronic device package can be another shape, suchas rectangular or irregular shape. Additionally, the pin count of 100solder columns in the depicted embodiment is exemplary and for purposesof explanation. Embodiments of electronic device packages can include anumber of pins more or less than this exemplary count.

Also, embodiments described herein may be utilized in combination withexisting column grid array apparatuses and techniques to provideenhanced reliability. For example, embodiments of an electronic devicepackage can include corner posts to secure the position of theelectronic device package substrate in relation to an underlying carriersuch as printed circuit board, as described in U.S. Pat. No. 6,646,356,and method of providing such posts, as described in U.S. Pat. No.6,680,217, the contents of which are incorporated by reference in theirentireties.

It will be apparent to those skilled in the art that various changes andmodifications can be made in the electronic device package method offabricating an electronic device package including a substrate and acolumn grid array of the present invention without departing from thespirit and scope thereof. Thus, it is intended that the invention coverthe modifications of this invention provided they come within the scopeof the appended claims and their equivalents.

The invention claimed is:
 1. An electronic device package, comprising: asubstrate; and wire columns arranged in plural groups about a neutralstress point of the substrate, each of the wire columns having a pair ofend portions, which are aligned with one another, wherein a height ofeach column is substantially uniform for the plural groups of wirecolumns, and a length of the wire columns increases for each group as afunction of each group's distance from the neutral stress point.
 2. Theelectronic device package of claim 1, wherein the neutral stress pointis located at the geometric center of the substrate.
 3. The electronicdevice package of claim 1, wherein the plural groups include at least asecond group surrounding a first group.
 4. The electronic device packageof claim 1, wherein each wire column in at least one of the pluralgroups includes a section bent in direction substantially parallel witha plane of a side of the substrate.
 5. The electronic device package ofclaim 4, wherein the at least one bent section has a “C” shape.
 6. Theelectronic device package of claim 1, wherein the plural groups includea first group located about the neutral stress point, and each wirecolumn in said first group is substantially linear.
 7. The electronicdevice package of claim 6, wherein the plural groups include a secondgroup, a distance from each wire column in the second group to theneutral stress point being greater than a distance from each wire columnthe first group to the neutral stress point, and each wire column insaid second group includes a bent section.
 8. Electronic device package,comprising: a substrate; and wire columns arranged in plural groupsabout a neutral stress point of the substrate, wherein a height of eachcolumn is substantially uniform for the plural groups of wire columns,and a length of the wire columns increases for each group as a functionof each group's distance from the neutral stress point, and wherein eachwire column in at least one of the plural groups is bent one mode past abuckling mode for that wire column's distance from the neutral stresspoint.
 9. Electronic device package, comprising: a substrate; and wirecolumns arranged in plural groups about a neutral stress point of thesubstrate, wherein a height of each column is substantially uniform forthe plural groups of wire columns, and a length of the wire columnsincreases for each group as a function of each group's distance from theneutral stress point, and wherein the wire columns are comprisedsubstantially of solder.
 10. The electronic device package of claim 9,wherein the neutral stress point is located at the geometric center ofthe substrate.
 11. The electronic device package of claim 9, wherein theplural groups include at least a second group surrounding a first group.12. The electronic device package of claim 9, wherein each wire columnin at least one of the plural groups includes a section bent indirection substantially parallel with a plane of a side of thesubstrate.
 13. The electronic device package of claim 12, wherein the atleast one bent section has a “C” shape.
 14. The electronic devicepackage of claim 9, wherein the plural groups include a first grouplocated about the neutral stress point, and each wire column in saidfirst group is substantially linear.
 15. The electronic device packageof claim 14, wherein the plural groups include a second group, adistance from each wire column in the second group to the neutral stresspoint being greater than a distance from each wire column the firstgroup to the neutral stress point, and each wire column in said secondgroup includes a bent section.